1. Field of the Invention
The present invention relates to an abrasion-resistant steel used for components being subjected to abrasive wear, slip wear, or impact wear caused by rocks, sands, or ores, and to a method for producing thereof, and particularly to an abrasion-resistant steel having excellent surface property and to a method for producing thereof.
2. Description of the Related Arts
Components of industrial machines, their parts, and transportation equipment which are employed in the field of construction, civil engineering, and mining are subjected to abrasive wear, slip wear, or impact wear. These components are made of abrasion-resistant steel to prolong the life of machines, equipment, and parts. It has been known that the increased hardness of steel increases the abrasion resistance of the steel. Accordingly, steel alloys including a large amount of alloying elements such as Cr and Mo are heat-treated to produce high hardness steels.
Japanese Patent Application Laid Open No. 62-142726, 63-169359, and 1-142023 disclosed abrasion-resistant steels to ensure high hardness. These abrasion-resistant steels have approximately 300 or more Brinell hardness at room temperature and have improved weldability, toughness, and bending workability.
The abrasion resistance required to recent abrasion-resistant steels has become much higher level than ever, and conventional method to simply improve the hardness can not satisfy the demand of fundamental improvement of abrasion resistance.
Common practice to increase hardness is the utilization of hardening by solution treatment or by precipitation. However, increase of hardness using an conventional technology of solution treatment, transformation, and precipitation hardening results in degradation of weldability and workability and further in significant cost increase owing to high alloying. Consequently, it is expected to be difficult for a practical use steel to considerably increase its hardness to improve the abrasion resistance.
In conventional steels, Ti is added beside the implementation of precipitation hardening treatment. The purpose of the addition of Ti to steel is mainly to fix N as TiN liable to combine with B in order to secure solution boron effective for quenching hardenability. For example, 0.01 wt. % N content needs about 0.04 wt. % or less Ti addition. When Ti is utilized in a form of TiN, it is sometimes used to prevent the growth of particles to coarse ones during the period of heating the steel to a high temperature. The necessary amount of Ti to prevent the particle growth is about 0.04 wt. % or less. Generally, however, TiN is considered to degrade ductility and toughness. Accordingly, N content of steel is lowered as far as possible at the steel-making process, and the addition of Ti is kept at a minimum level to reduce the amount of generated TiN. The amount of Ti addition is 0.02 wt. % or less.
The above described object of Ti addition and amount of added Ti in conventional technology is summarized below.
(a) With the precipitation hardening using very fine TiC precipitates, the hardness of steel matrix is increased. Then, with the increase of hardness, the abrasion resistance is improved. PA1 (b) In the case that TiN is utilized to improve the hardenability of B or to enhance the generation of fine crystal grains, the Ti addition is kept at approximately 0.04 wt. % or less, and practically at approximately 0.02 wt. % or less.
Since abrasion-resistant steels contact with rocks and ores on their surface under a practical use condition, any surface defect may result in cracks. Accordingly, the surface property is an important factor because cracks occur from surface defects.